Thanks to some excellent advice from my good friend Jim The Brewer of Brentwood Brewing Co, I've now modified my boiler to make sure the hop oils from the late additions are not vented off.
All I've done really is to drill six 10mm holes so that the 3/8" hose that connects my triple stainless steel immersion chillers can remain in the boiler while the lid is put on.
A slightly surprising by product of the lid is the decreased cooling time. This time my wort cooled from 95c to 72c in 8 minutes. At this time I turned the chillers off, added the steep hops and popped the lid on for 10 mins before putting cooling down to 35c in a total of 40 minutes.
The temperature of the water exiting the chillers was upped from 50c to 58c. Of course the real test will be how hoppy this beer - an American IPA - turns out to be.